Interpretive Summary: Long-term evaluations of current and potential biofuel production systems are needed to determine environmental benefits and overall sustainability. Grain and biomass yields and composition, soil carbon changes, and production input data were obtained to estimate ethanol yields per acre and greenhouse gas emissions from a long term corn and switchgrass replicated field trial in the western Corn Belt USA. From this study, it was determined that continuous no-till corn with best management practices that result in soil carbon sequestration and switchgrass grown for bioenergy could result in significant net greenhouse gas reductions compared with existing transportation fuels. Under the best management conditions in the study, both switchgrass and corn harvested for both grain and stover were net energy positive. It also was determined that co-locating and integrating cellulosic ethanol biorefineries with existing corn grain ethanol facilities would result in improved environmental benefits and net energy efficiency as compared to the current corn grain ethanol system.

Technical Abstract:
Grain and biomass yields and composition, soil carbon changes, and production input data from a long-term continuous corn and switchgrass replicated field trial in the western Corn Belt USA were used to estimate energy yields and greenhouse gas (GHG) emissions. Corn was produced without tillage with different N fertilizer and stover harvest treatments. We determined that with good management practices, continuous no-till corn and switchgrass grown for bioenergy could result in near-term net GHG reductions of -29 to -396 grams of carbon dioxide equivalent emissions per megajoule of ethanol per year as a result of direct soil carbon sequestration and the use of integrated grain and biomass biofuel conversion pathways. Switchgrass using best management practices produced 3919 ± 117 liters of ethanol per hectare and had petroleum offsets of 74 ± 2.2 gigajoules of petroleum per hectare which was similar to the best corn system (no-tillage, optimal N rate, grain and residue harvest). Co-locating and integrating cellulosic and corn grain ethanol biorefineries would significantly improve net energy efficiency as compared to the current corn grain ethanol system.